2:45 PM - 3:00 PM
[BPT05-17] Estimation of 3.2 Ga seawater-hydrothermal environment from sulfur isotopic analyses of barite crystals in Dixon Island Formation, Western Australia
Keywords:Archean, sulfur isotopes, barite, SIMS
In this way, δ34S of past sulfate minerals is a good proxy for redox state and microbial sulfate reducing systems in the Precambrian. However, reports of δ34S of Archean sulfate are scarce and localities and ages of research are partial. Therefore, we focused on newly discovered sedimentary barite (BaSO4) layers from the 3.2Ga Dixon Island Formation, which is considered to have been deposited in a relatively deep open sea environment (Kiyokawa et al., 2006).
The Dixon Island Formation is located in the coastal Pilbara terrane, Western Australia and shows low metamorphic grade (Kiyokawa and Taira, 1998). Barite layers alternate with black chert layers in the Black Chert Member of the Dixon Island Formation that overlies hydrothermal vein networks. Barite is considered to have formed during precipitation of black chert. Though most of them are silicified (Kiyokawa et al., 2006), there exist small crystals of barite (less than 200 μm in diameter) which are regarded to be remnants of original barite. We crushed three rock samples from different horizons, separated twelve fine barite grains in total, and performed micro-meter scale δ34S analyses using a NanoSIMS.
We used five sedimentary barites as working standards which are considered to have δ34S homogeneity in each crystal determined by an IsoPrime. For measuring samples, we performed raster analyses of two or three spots for each crystal, and values were averaged. As a result, we obtained scattered δ34S values of -2.1 to +18.7‰ (Avg.=+6.5‰, 1σ=6.3‰) from twelve crystals. On the other hand, averages in each rock sample were +3.4, +7.8 and +8.4‰. Measurement errors were ±0.87‰ to ±3.72‰.
Average δ34S values for each rock sample are similar to literature values of Archean sulfate (+5-10‰, Canfield and Farquhar, 2009). Meanwhile, focusing on the range of values of each crystal, lower ones were near δ34S of mantle-originated sulfur (ca. 0‰), which may reflect δ34S of hydrothermal-originated sulfate. Besides, higher ones were near δ34S of modern seawater sulfate (+22‰) and thus higher than Archean sulfate. There are two possible causes of high δ34S: 1) extreme microbial sulfate reduction in an environment closed with respect to sulfate (Rayleigh fractionation) or 2) hydrothermal fluid supplying isotopically heavy sulfate.
This study is the first attempt of in situ δ34S analyses for Archean barite microcrystals. We detected isotopic heterogeneity in individual barite crystals within three single barite beds. Conventional combustion method for S isotope analysis may mix this heterogeneity and provide us with averaged values. There is a possibility that δ34S dispersion in each barite bed shows isotopic heterogeneity of seawater sulfate at that time.